This research proposal involves the understanding and quantitative design of cycloadditions suitable for biomolecule labeling under physiological conditions. We will: 1. perfect computational methods to be used for accurate and routine predictions of rate constants of cycloadditions in aqueous media; 2. complete the computations of rate constants of all possible cycloadditions between cycloaddends that are known to be bioorthogonal cycloaddition reagents with a given cycloaddition partner but have not been explored in reactivities towards other cycloaddends. The resulting matrix of predicted rates for a series of dienes and 1,3-dipoles with a series of alkenes and alkynes will guide the discovery of additional useful bioorthogonal and mutually orthogonal reactions; 3. develop generalized design principles for new reagents and orthogonal reaction pairs; 4. conceive of new cycloaddition components and test them computationally in order to determine promising reactant pairs for bioorthogonal cycloadditions. 5. Compute fluorescence wavelengths and quantum yields for newly designed fluorogenic probes. The results will enhance the monitoring of biological processes and disease states.